Item — HN Reader

> How much dangerous are the neutrons from such device? Similar to radiation, worst?

Neutrons are interesting in that they dump all their energy into a very small region. Instead of losing energy continually and gradually, they travel a ways and then dump all their energy into one spot. This makes them potentially very useful for things like cancer treatment (you can select neutrons to ensure they deliver most of their energy into the cancer, rather than evenly along their travel path). I suspect this effect means that, for reactors, the wear and tear is more localized, but of a larger amplitude.

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> I suspect this effect means that, for reactors, the wear and tear is more localized, but of a larger amplitude.

Not exactly. The initial collisions do indeed cause localised damage cascades, but the overall effect of a large neutron flux is embrittlement of the material. This is a problem already in fission reactor vessels, but the neutron fluxes are a whole order of magnitude larger in fusion reactors, and it's a very difficult problem to solve. To my knowledge, current materials can only just about deal with with the neutron fluxes in standard fission reactors, with no current materials being capable of withstanding neutron fluxes in a fusion reactor over any long (i.e. commercial) timescale at present. It would be a big achievement and a big jump in materials science to discover such a material, completely separate of any fusion project.

(Disclaimer: this is not remotely my field, but I have looked at some of this stuff in the past, and been to talks about ITER.)

Do the neutrons necessarily come off in all directions, or can the reactor be designed so that the neutrons tend toward specific directions?

It's a simple geometric analysis: neutrons can't be deflected, so whatever concentration of reacting mass you have emits neutrons isotropically. So to my knowledge no (you can only radiate more in one direction if the reaction is more extended in the perpendicular direction).

http://www.thomasnet.com/products/neutron-shielding-74851395...

The majority of those suppliers provide boron-impregnated plastic, which is fine for e.g. shielding spallation neutrons from a medical linac head. Reactor shielding is orders of magnitude higher, so material damage is a real concern. Not to say that people can't or don't use that material, but it is not the most cost effective stuff.

Frankly, the best neutron shielding in the world, on a per-cost basis, is water with borax. I don't know why people don't use this more. We used to use stacked bags of borax as neutron shielding when I built a fusion reactor (non-self sustaining, of course), and we had more than enough shielding to handle 2.45 MeV neutrons for under $1000.

So in this reactor design, the neutron shield would be quite thin, if that drawing is to scale I'd say like 30-50cm? in that case water in borax wouldn't be dense enough right?

A cool thing about the shield being liquid is that you could theoretically replace it while it's running. It could make the neuron shield double as the heat transfer medium too.

Of course I'm a total layman so this is just highlevel blabbering.

According to "neutrons make other things radioactive", does it make your water with borax radioactive too?

Are you sure you're not mixing up neutrons and protons? Here are some typical depth dose curves from google images(http://www.nap.edu/books/11976/xhtml/images/p20014b2bg205001...). Neutrons, being neutral particles, don't exhibit the Bragg Peak (large increase in energy deposition at the end of the particle's track) that you get for heavy charged particles.

I was pretty sure neutrons beahved that way (recalling from a course I took in grad school), but it has been a few years, and I do not have references handy. I am not entirely sure what the y-axes are, on the plot you linked, so I am not sure how it correponds to what I was saying.

The do not. Protons have a quite definite range, and can be controlled in the way you suggest. This is because they lose energy primarily by scattering (much lighter) electrons out of their path. This means protons have relatively straight paths and the dynamics of the electromagnetic interaction gives an energy deposition curve that is sharply peaked at the end.

Neutrons slow down via interaction with nuclei (all of which except hydrogen are heavier) so they lose energy slowly and scatter all over the place. They have no definite range (search for "fermi age theory" to get a rough idea of the distribution) and can't be meaningfully beamed (unless they are ultra-cold, which is not relevant to fusion power.)

I've made a longer comment above that goes into neutron physics in a little more detail.

I should note that my response here was incorrect. Thanks to loarake and tjradcliffe for the correction.